--- a/mercurial/configitems.py	Mon Nov 21 15:04:42 2022 -0500
+++ b/mercurial/configitems.py	Sun Nov 20 23:09:12 2022 -0500
@@ -1786,6 +1786,13 @@
 )
 coreconfigitem(
     b'merge-tools',
+    br'.*\.regappend$',
+    default=b"",
+    generic=True,
+    priority=-1,
+)
+coreconfigitem(
+    b'merge-tools',
     br'.*\.symlink$',
     default=False,
     generic=True,